1. Field of the Invention
The present invention relates to lightning strike protection (LSP) in composite structures. More particularly, the invention provides a method and system for examining a repair to a composite structure with LSP by imaging for detecting thermal gradients in an inductively heated LSP mesh at the periphery of the repair.
2. Description of the Related Art
Aircraft are being designed and built with greater percentages of composite material. For example, the Boeing 787 currently under development by the assignee of the present invention will have more than 50% composites for its primary structure. Although composites are lighter and have better mechanical and fatigue properties than traditional aluminum, they are less electrically conductive, and have limited electromagnetic shielding, causing reduced current dissipation at the lightning strike attach point. Compared to traditional aluminum, composites are therefore subject to greater damage due to lightning strikes.
When lightning hits an aircraft, a direct, conductive path on the skin of the aircraft will allow the electricity to travel along the skin, and exit at some other location. Without an adequate conductive path, arcing and hot spots can occur that will char, delaminate, or penetrate the skin, reducing the load-bearing characteristics of the structure. Also, the low shielding capability of composite materials increases the likelihood that electrical circuits will be affected by the lightning strike. One way to protect composite skins from lightning strike damage is to include conductive lightning strike protection (LSP) either in or on the composite skins of an aircraft. For composite components that experience a lot of lightning strikes, such as radomes, rudder tips, elevator tips and aileron tips, LSP often consists of aluminum frames or strips bonded/fastened to the structure and electrically bonded to it. For larger areas, this LSP method is not practical or cost effective. Other methods, such as expanded aluminum foil bonded to the surface, aluminum flame spray coatings, or aluminum coated glass fabric used as a surface layer can all be used for LSP. An approach being developed for 787 is a copper mesh or grid embedded into the skin or backed by film adhesive that is bonded to the skin surface.
A problem arises when a composite skin containing lightning strike protection is damaged and has to be repaired. After the damage is cut away, a composite patch (either pre-cured or cured in-place) is used to replace the material. Even if the patch contains LSP, it is critical to ensure a good conductive path between the patch and surrounding material. Recent test have demonstrated that the patch will be damaged or destroyed if lightning current cannot pass through it. The problem of applying patches that do not reduce, but rather extend the useful LSP area is a significant one.
Until recently, only direct electrical conductivity measurements to verify good contact between the LSP on the structure and the applied patch were available. Measurements were taken all around the patch edge. These were tedious, and did not have high efficacy. The same issue arises for boundaries between individual sections of LSP material during manufacturing.
A recent invention disclosed in U.S. patent application Ser. No. 11/266,052 filed on Nov. 3, 2005 entitled SYSTEMS AND METHODS FOR INSPECTING ELECTRICAL CONDUCTIVITY IN COMPOSITE MATERIALS having a common assignee with the present application is one approach for addressing this problem.